1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Complex number

  1. Nov 7, 2012 #1
    Find (2+6i)^(1/3). Then we need to let z=a+bi, then z^3....... Then we will get a=? And b=?
    Why we can do z=a+bi assumption? Any prove show what we assume is correct and how do we know the answer come out is correct?
  2. jcsd
  3. Nov 7, 2012 #2
  4. Nov 8, 2012 #3
    Sorry, can you tell what is the subtopic of my question in the reference given. Thanks
  5. Nov 8, 2012 #4


    User Avatar
    Science Advisor

    I'm not sure what you mean by "z= a+ bi assumption". Typically that is NOT an "assumption", it is one way of defining complex numbers.

    A more formal definition, from which you can then derive "z= a+ bi" is this:
    The complex numbers is the set of all pairs of real numbers, (a, b), with addition defined by (a, b)+ (c, d)= (a+ c, b+ d), and with multipication defined by (a, b)(c, d)= (ac- bd, bc+ ad). One can then show that the usual "properties" of arithmetic (commutativity, associativity, etc.) hold. One can also show that the subset of all pairs in which the second member is 0 is "isomophic" to (identical to) the set of real numbers: (a, 0)+ (b, 0)= (a+ b, 0+ 0)= (a+ b, 0) and (a, 0)(b, 0)= (ab- 0(0), a(0)+ 0(b))= (ab, 0). We can "identify" complex numbers of the form (a, 0) with the real number, a.

    One can also show that [itex](0, 1)^2= -1[/itex]: (0, 1)(0, 1)= (0(0)- 1(1), 0(1)+1(0))= (-1, 0) which, as above, we identify with the real number -1. We define "i= (0, 1)" so that [itex]i^2= -1[/itex]. And, then, we can write (a, b)= (a, 0)+ (0, b)= (a, 0)+ (b, 0)(0, 1)= a+ bi.
  6. Nov 15, 2012 #5
    Thank you.
    Can I say all the number actually is the combination of real and imaginary number.
    As you say" We can "identify" complex numbers of the form (a, 0) with the real number, a"
  7. Nov 16, 2012 #6


    User Avatar
    Science Advisor

    Yes, that is write. By any definition of "complex" number, we can write any complex number in the form z= a+ bi for some real numbers "a" and "b". That form is also "unique" that is, if z= a1+ b1i and the same z= a2+ b2i, we must have a1= a2 and b1= b2.

    Now, in this problem, attempting to find (2+ 6i)1/3, they are suggesting that you write the answer as "a+ bi", find (a+ bi)3 and set the "real" and "imaginary" parts equal. For example, (a+ bi)2= (a+ bi)(a+ bi)= a2+ 2abi+ b2i2= (a2- b2+ 3abi. If we wanted to find (1+ i)1/2, we could now set s2- b2= 1 and ab= 1, giving two equations to solve for a and b.
  8. Nov 16, 2012 #7

    Thanks a lot :!!)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook